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He said "our country will be better off" if Congress renders its own opinion.

At the same time, he challenged politicians to consider "what message will we send to a dictator" if he is allowed to kill hundreds of children with chemical weapons without suffering any retaliation.

US Senate Republican leader Mitch McConnell said President Obama's role as Commander-in-Chief is always strengthened when Congress is asked for support.

US politicians will return to session on 9 September.

Earlier the United Nations vehemently rejected suggestions that it was somehow stepping aside to allow US air strikes on Syria and said its humanitarian work in the conflict-ravaged country would continue.

"I have seen all kinds of reporting suggesting that the departure of the chemical weapons team somehow opens a window for military action of some kind," UN spokesman Martin Nesirky told reporters.

"Frankly, that's grotesque, and it's also an affront to the more than 1,000 staff, U.N. staff, who are on the ground in Syria delivering humanitarian aid and who will continue to deliver critical aid," he said.

Diplomatic sources said it could take two weeks before UN inspectors' tests into chemical weapons in Syria will be known.

The US says 1,400 died in an attack by Syrian forces which Damascus denies.

British Prime Minister David Cameron said he supported Mr Obama's position on Syria.

But Mr Cameron's plans for the UK to join a potential military strike were thwarted on Thursday when parliament narrowly voted against a government motion to authorise such action in principle.

Meanwhile, Russian President Vladimir Putin said today he was 'totally surprised' by the Westminster vote but said it would be 'extremely sad' if the US took unilateral action against Syria.

He said it would be 'utter nonsense' for the Syrian government to use chemical weapons.

Mr Putin said that the US should present evidence of chemical weapons use to the UN Security Council.

He also urged Mr Obama as a Nobel Peace Prize winner to consider the potential victims of any US attack on Syria.

A French presidential official said that France will wait for discussions in the US Congress and French parliament on Syria before making a decision on military intervention.

The official said President Francois Hollande spoke with President Obama today and the two agreed to act together on Syria.

France's parliament was already planning to convene on Wednesday about Syria, but Mr Hollande does not need its permission to intervene militarily.

President Putin said that next week's G20 summit in St Petersburg could serve as a platform to discuss the Syria crisis.

US officials have said that President Obama will discuss the case for action on Syria with world leaders at the G20 summit in Russia next week.